1. Field of the Invention
The present invention relates to a catalytic composite for deodorizing odorous gases which oxidizes nitrogen-containing odorous compounds such as ammonia and amines to decompose them, and more particularly, it relates to a catalytic composite for deodorizing odorous gases which can attain excellent deodorizing or oxidation activity at ordinary temperature. It also relates to a method for preparing the catalytic composite, and a deodorizing apparatus using the catalytic composite.
2. Description of the Prior Art
As a deodorizer for removing odors, adsorbents such as active carbon which physically adsorb odorous substances are widely used. But these adsorbents cannot be used after the odorous substances are adsorbed onto the entire surface thereof. At present, there are very few deodorizers which can be used repeatedly.
In recent years, there has been proposed an ozone-utilized deodorizing apparatus which employs ozone which is known to have excellent oxidation activity and also employs an ozone-decomposing catalyst for decomposing excessive ozone through oxidation. This deodorizing apparatus can be used repeatedly for removing odors. However, the ozone-utilized deodorizing apparatus requires an ozone generator, and also requires a system for reducing the leakage of ozone to a level of 0.1 ppm or lower because ozone is harmful to the human body. The necessity of such auxiliary devices makes the whole deodorizing apparatus larger in size and increases the production cost thereof.
On the other hand, as a deodorizer for industrial use, a variety of catalysts for the catalytic oxidation of odorous substances have been put into practical use. Such oxidation catalysts can be used repeatedly because they decompose the odorous substances into odorless substances. But the oxidation catalysts require heating to a temperature of at least 200.degree. C. in order to attain their catalytic activity. This necessitates the use of a heater in combination with the catalyst. Therefore, there has been a demand for a catalyst which can attain catalytic activity at low temperatures.
As a catalyst capable of attaining catalytic activity at low temperatures, hopcalite containing manganese oxide and copper oxide as its main components has been put into practical use. However, this catalyst has a disadvantage in that its low-temperature catalytic activity is rapidly lowered by moisture.
As another catalyst having low-temperature catalytic activity, a catalyst containing a metal oxide and gold has been proposed (Japanese Laid-Open Patent Publication No. 60-238148). This gold-metal oxide catalyst is supported on a carrier such as alumina to form a catalytic composite, and used as an oxidation catalyst for CO. The gold-metal oxide catalyst exhibits excellent efficiency in the combustion of carbon monoxide at a low temperature of, for example, -30.degree. C. However, the catalytic composite prepared by this method cannot attain sufficient efficiency in removing odors. The reason is that the porosity of the carrier is not sufficient to provide the required oxygen. For removing odors, a temperature of 200.degree. C. or more is required.
The above-mentioned type of catalytic composite which includes a carrier and a gold-metal oxide catalyst supported thereon can be prepared by coprecipitation (U.S. Pat. No. 4,698,324). In this method, a ceramic carrier is immersed in an aqueous solution containing a water-soluble salt of gold and another metal, and also containing urea and/or acetamide, after which catalytic components, i.e., the gold and metal oxide, are allowed to precipitate on the carrier.
In the preparation of such a catalytic composite, the catalytic components are required to be firmly supported on the carrier made of an inorganic material so as to ensure sufficient catalytic activity in the resultant catalytic composite. However, in the catalytic composite prepared by the above-described conventional method such as immersion, the catalytic components tend to drop from the carrier due to the poor adhesion therebetween. This lowers the catalytic activity of the catalytic composite.
In order to obtain a catalytic composite with high activity, there has been proposed another method for preparing this type of catalytic composite (Japanese Laid-Open Patent Publication No. 1-94945). In this method, first, a metal oxide such as an iron oxide
P9619 is supported on a carrier made of alumina, silica, zeolite, titania, or the like so that the content of the metal oxide in the resulting carrier is 20 percent by weight or more. Then, the thus obtained carrier is immersed in an alkaline solution of pH 7.5 or more. While the pH of the alkaline solution is maintained in the range of 7.5 to 9.5, a solution of a gold compound such as chloroauric acid is added to the alkaline solution, so that the gold compound precipitates on the carrier. The carrier having the gold compound thereon is then calcined, resulting in a catalytic composite. The catalytic activity of the resulting catalytic composite including the carrier and the gold-metal oxide catalyst supported thereon is as high as that of a gold-metal oxide catalyst used alone without a carrier. However, this method requires a complicated production process as described above and accordingly requires a large amount of equipment, thereby increasing the production cost.
In general, in order to improve the catalytic performance of a catalytic composite for deodorizing odorous gases, the carrier for supporting catalysts thereon is so formed as to have a large surface area in contact with air. For example, a ceramic such as silica, alumina, zeolite, cordierite, or mullite is formed into a honeycomb structure in order to obtain a large area on which a catalyst can be carried, and used as a carrier. Generally, the catalyst is applied to the carrier by spraying or immersion. In this case, however, the carrier of this type is usually obtained by a sintering with high temperature, so that the degree of porosity is low. Accordingly, catalytic components can be supported only on the surface of the ceramic carrier, so that the catalytic activity is low. Furthermore, when zeolite is used as a carrier, the carrier has poor mechanical strength and is expensive.